In dialysis, the blood of the patient flows through a tubing system of an extracorporeal blood circulation. On the one hand, the extracorporeal guiding of blood enables easy access to the patient's blood so as to determine analytical parameters from the same. On the other hand, the extracorporeal guiding of blood constitutes a risk factor in dialysis treatment, as in this way undesired foreign matter such as impurities or air may be supplied to the patient.
The fields of application of most various sensor systems focusing on the detection of such foreign matter are correspondingly manifold. Accordingly, special attention is paid to sensor methods which are capable of analyzing the blood and, respectively, the content of the blood tubing system in a contactless and non-destructive manner. Contactless in this context means that the blood does not contact any material other than the standard material of the tubing system. Thus, on the one hand, the occurrence of additional centers of coagulation or inflammation is avoided. On the other hand, manufacturing costs are minimized as it is not necessary to combine different materials and structures. Non-destructive in this context means that the measuring technique does not affect, weaken or even destroys the cellular or molecular components of the blood. This is especially important as the formation of red blood cells is impaired in dialysis patients and each loss of the same may permanently impair the health condition and the well-being of the patient.
It is known that optical technologies are suited for contactless and non-destructive analysis of substances. Light may interact with the analyte without any direct contact between the analyte and the light source or detector and only under extreme conditions will result in a damage of cells, for example. Such extreme conditions may be easily avoided by a smart design of the sensor.
The use of optical sensors in dialysis is known. These include e.g. hematocrit sensors (on the basis of red and infrared light), red detectors (indicating the presence of blood in the blood tubing system) and blood leakage detectors (measuring reddening in dialysis solution in the dialysis solution circuit) or analytical sensors in the dialysis solution circuit for real-time monitoring, for example those permitting determination of a dialysis dose during therapy. Said sensors measure at least quasi balanced states, i.e. they indicate a concentration, a coloring or the presence of a substance in the blood, for example. For this, the entirety of the light incident on the detector is analyzed and processed to form a measuring value. For this reason, no special demands are made to the lighting system of said sensors. In general, said sensors are neither adapted to make any statements about short-term disturbances and, respectively, are even negatively influenced by the same, nor are they capable of determining a measuring value in a space-resolved manner, i.e. at various locations inside the tube.
In several applications such as the detection of air bubbles or dissolved impurities, for example, spatial resolution is favorable in order to identify and quantify said air bubbles or impurities (air bubbles e.g. as to volume, impurities as to size and structure). For space-resolved measurement detector arrays which are capable of reproducing spatial information by the combination of a plurality of pixels are required. A simple example of such measuring system based on spatial resolution are cameras imaging the system to be examined onto a CMOS or CCD element and thus providing the spatial information for analysis. Other than in the case of optical sensors, in such imaging measuring systems a homogenous illumination of the entire detector array is an important characteristic. If the illumination of one pixel of the detector array is weaker than that of another one, this is traced back to local interaction between the light and the analyte. If, instead of this, a different illumination is caused by inhomogeneous distribution of the incident light, this results in misinterpretation of the measurement.
For measurement on a tube, further criteria have to be observed. A tube as a substantially cylindrical object may act, on the one hand, as a lens resulting in different illumination of the detector array, as is schematically shown in FIG. 1. The lens effect shown in FIG. 1 results in inhomogeneous light distribution on a detector unit when a tube is transilluminated. The effect is moreover dependent on whether a fluid and which type of fluid is provided therein.
On the other hand, in the case of transillumination of the tube section, the optical paths of the light through the medium inside the tube are different (see FIG. 2). This, too, usually results in inhomogeneous illumination of the detector array, as in the case of homogeneous transillumination of a tube the light beams at the rim of the tube interact less strongly than light beams in the middle of the tube with the medium inside the tube. This equally results in an inhomogeneous light distribution on the detector unit, as light penetrating at the rim of the tube is absorbed less than light penetrating the central area of the tube. Differences in illumination analogous hereto may occur when the illumination system consists of a point light source or few point light sources such as arrays of light-emitting diodes or LED.
As afore-described, in prior art merely homogeneous illumination of a flat surface has been described.